The present disclosure relates to a valve for a hydraulic damper including a hydraulic damper for use in wheeled vehicle suspension.
Hydraulic dampers which are used to control dynamic systems generally utilize a piston adapted to be slideably carried within a cylindrical main body and peripherally sealed to its walls. The hydraulic damper is configured to be attached to a shaft assembly. The piston divides the cylindrical main body into two hydraulic chambers that are connected by restrictive passages which retard the rate of fluid flow between the two hydraulic chambers when the shaft assembly moves relative to the main body. In this manner the damper's core operating characteristic, defined by the relationship between pressure and flow, is dictated by the geometric configuration of restrictive passages between the two hydraulic chambers
If the restrictive passages are simply configured as fixed orifices then the pressure generated across the damper piston increases as the square of the hydraulic flow through the orifices. Unfortunately this squared law pressure-flow relationship is not a desirable characteristic for controlling the majority of dynamic systems. In the case of an automotive suspension system the damper is normally referred to as a shock absorber and the pressure-flow characteristic is directly proportional to the shock absorber's defining force-velocity relationship which is generally required to be linear or even somewhat digressive. The method of achieving damper characteristics that differ from the basic fixed orifice square law is to vary the area of the orifice in a predetermined relationship to the pressure across the piston.
The most common variable orifice damper valve arrangement consists of a stack of compliant plates fixed in place over an array of passages that connect the two hydraulic chambers either through or around the piston. The pressure across the piston imparts a load on the plates causing them to deflect which in turn uncovers the passages and creates a path for the damper's hydraulic fluid. The magnitude of the deflection of the plates varies in proportion to the pressure across the piston and so creates a form of variable orifice. U.S. Pat. No. 2,748,898 to DeCarbon is the earliest reference to such an arrangement and describes a double acting shock absorber in which the piston is configured with an arrangement of passages that are sealed by resilient leaf elements that are stressed and resiliently bent by fluid issuing under pressure from the passages. The '898 patent also details a unique, but now widely utilized, method of arranging the passages and two sets of leaf elements, above and below the piston, so as to facilitate independent and possibly asymmetric pressure-flow characteristics in the two different operating directions.
The most significant limitation of using compliant plates to create a variable orifice damper valve is that the pressure-flow characteristic is highly dependent on the deformed shape of the compliant plates which in turn is extremely sensitive to plate thickness, plate material properties, dimensional tolerance of the plate shape, assembly process, friction between the plates in a stack, preload on the plates in a stack, locational tolerance of the passages relative to the plates, dimensional tolerance of the passage cross sections and cleanliness of the assembly. These sensitivities ultimately present a significant challenge to achieving a desired pressure-flow characteristic or when trying to match the characteristic of two dampers. An additional disadvantage of the compliant plate arrangement is that the pressure-flow characteristic cannot be easily predicted using mathematical techniques due to its complex operating mechanism. Another shortcoming of this configuration is that the pressure-flow characteristic tends to diverge from its original curve over time due to the compliant plate material becoming fatigued and losing its stiffness and strength as well as small particles, created from seal, piston and shaft wear, become trapped between the plates.
U.S. Pat. No. 5,547,050 issued to Beck illustrates the complexity associated with manufacturing and assembling a damper that utilizes compliant plates as a variable orifice. The '050 patent describes a method of attaching the plates and piston to a shaft to overcome some of the dimensional limitations associated with the arrangement. However, although the assembly approach outlined by the '050 patent eliminates the tolerances related to the compliant plate attachment it does not improve the variation associated with the dimensional accuracy of the plates themselves or the divergence from the original pressure-flow characteristic that occurs over time. Moreover, the '050 patent does not describe an arrangement for which the characteristic can be mathematically predicted.
U.S. Pat. No. 5,709,290 issued to Ekert et. al. describes a method of providing compression and rebound stop surfaces which uniformly support the compliant plates in their deflected state at both limits of deflection travel. The compliant plates of the '290 patent are prevented from yielding to a deformed condition which can significantly alter the designed-in performance characteristics of the damper assembly. This stop surface arrangement considerably improves the damper's ability to maintain its original pressure-flow characteristic over time. However, this system is particularly sensitive to detailed tolerances such that minor variations in specific design features may result in significant, undesirable changes in performance characteristics.
The limitations of variable orifice damper valves that utilize compliant plate stacks has been recognized but although numerous alternatives have been suggested, and appear in the prior art, this arrangement remains the absolute dominant approach to providing the desired pressure-flow characteristics in shock absorbers as used in automotive suspension systems.
U.S. Pat. No. 6,311,812 to Sönsteröd et al offers an alternative to the compliant plate approach by describing a poppet style pressure regulator that utilizes pressure balancing across the poppet to control the area of the resulting annular orifice. The shape of the front side of the poppet can be varied to control the pressure balancing. In this manner the overall pressure-flow characteristic of the pressure regulator and ultimately a damper that utilizes the device is controlled by the varying area of the annular orifice. Although the '812 patent overcomes many of the tolerance sensitivity problems associated with compliant plate variable orifice damper valves its basic configuration is limited by only offering an annular orifice hydraulic restriction. This limitation is overcome in alternative embodiments of the present disclosure but only through the addition of significant complexity which once again introduces additional sensitivity to manufacturing tolerance. However, the most significant limitation of the valve arrangement of the '812 patent is that the valve arrangement is uni-directional. For the pressure regulator of the '812 patent to be utilized in a double acting shock absorber, an array of one way ball valves are implemented to act in both compression and rebound directions. This limits the pressure-flow characteristic of the damper to be identical in the compression and rebound directions which is rarely desirable. Additionally the pressure regulator of the '812 patent is large and complex and cannot be contemplated to be integrated into a damper piston. Finally, similar to the compliant plate configurations, the '812 patent does not describe an arrangement for which the pressure-flow characteristic can be mathematically predicted.
An effective but complex solution to the tolerance sensitivity problems that exist in passive variable orifice valves is described in U.S. Pat. No. 5,996,745, issued to Jones et al. The '745 patent claims a damper valve for controlling the pressure-flow and therefore force-velocity characteristic of a shock absorber that consists of a bender with a piezoelectric material embedded within it. The bender is used in a similar manner to the compliant plates of a conventional damper valve but by supplying a voltage across the piezoelectric material the stiffness of the bender is changed and the pressure required to deform the bender is modified. An electronic sensor is utilized to measure the velocity of the piston and the voltage supplied to bender is varied in relationship to the measured velocity. In this manner the stiffness of the bender is made dependent on the velocity of the damper and the force-velocity and therefore pressure-flow characteristics actively controlled using a feedback system. Although the piezoelectric material based variable orifice valve of Jones can overcome the tolerance limitations of passive damper valves the associated complexity and cost is prohibitive. Additionally, the '745 patent does not describe an arrangement for which the pressure-flow characteristic can be simply mathematically predicted.
Accordingly, it is desirable to provide a damper valve that eliminates the complexity associated with the existing styles of variable orifice arrangements yet offers a simple configuration that provides a mathematically predictable, repeatable and robust pressure-flow characteristic.